/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*
 * Forked from https://github.com/codahale/metrics
 */

package org.apache.solr.util.stats;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

import static java.lang.Math.exp;

/**
 * An exponentially-weighted moving average.
 *
 * @see <a href="http://www.teamquest.com/pdfs/whitepaper/ldavg1.pdf">UNIX Load Average Part 1: How
 *      It Works</a>
 * @see <a href="http://www.teamquest.com/pdfs/whitepaper/ldavg2.pdf">UNIX Load Average Part 2: Not
 *      Your Average Average</a>
 */
public class EWMA {
  private static final int INTERVAL = 5;
  private static final double SECONDS_PER_MINUTE = 60.0;
  private static final int ONE_MINUTE = 1;
  private static final int FIVE_MINUTES = 5;
  private static final int FIFTEEN_MINUTES = 15;
  private static final double M1_ALPHA = 1 - exp(-INTERVAL / SECONDS_PER_MINUTE / ONE_MINUTE);
  private static final double M5_ALPHA = 1 - exp(-INTERVAL / SECONDS_PER_MINUTE / FIVE_MINUTES);
  private static final double M15_ALPHA = 1 - exp(-INTERVAL / SECONDS_PER_MINUTE / FIFTEEN_MINUTES);

  private volatile boolean initialized = false;
  private volatile double rate = 0.0;

  private final AtomicLong uncounted = new AtomicLong();
  private final double alpha, interval;

  /**
   * Creates a new EWMA which is equivalent to the UNIX one minute load average and which expects
   * to be ticked every 5 seconds.
   *
   * @return a one-minute EWMA
   */
  public static EWMA oneMinuteEWMA() {
    return new EWMA(M1_ALPHA, INTERVAL, TimeUnit.SECONDS);
  }

  /**
   * Creates a new EWMA which is equivalent to the UNIX five minute load average and which expects
   * to be ticked every 5 seconds.
   *
   * @return a five-minute EWMA
   */
  public static EWMA fiveMinuteEWMA() {
    return new EWMA(M5_ALPHA, INTERVAL, TimeUnit.SECONDS);
  }

  /**
   * Creates a new EWMA which is equivalent to the UNIX fifteen minute load average and which
   * expects to be ticked every 5 seconds.
   *
   * @return a fifteen-minute EWMA
   */
  public static EWMA fifteenMinuteEWMA() {
    return new EWMA(M15_ALPHA, INTERVAL, TimeUnit.SECONDS);
  }

  /**
   * Create a new EWMA with a specific smoothing constant.
   *
   * @param alpha        the smoothing constant
   * @param interval     the expected tick interval
   * @param intervalUnit the time unit of the tick interval
   */
  public EWMA(double alpha, long interval, TimeUnit intervalUnit) {
    this.interval = intervalUnit.toNanos(interval);
    this.alpha = alpha;
  }

  /**
   * Update the moving average with a new value.
   *
   * @param n the new value
   */
  public void update(long n) {
    uncounted.addAndGet(n);
  }

  /**
   * Mark the passage of time and decay the current rate accordingly.
   */
  public void tick() {
    final long count = uncounted.getAndSet(0);
    final double instantRate = count / interval;
    if (initialized) {
      rate += (alpha * (instantRate - rate));
    } else {
      rate = instantRate;
      initialized = true;
    }
  }

  /**
   * Returns the rate in the given units of time.
   *
   * @param rateUnit the unit of time
   * @return the rate
   */
  public double getRate(TimeUnit rateUnit) {
    return rate * (double) rateUnit.toNanos(1);
  }
}
